Shell Programming and Scripting

Okay, I now understand how a simple here-document works. However, I'm still having trouble understanding why there is a EOP "here document" within a another EOF here-document; and, how it relates to the backtick in the script.

There is everything within:

Code:

<< EOF
$(
<< EOP

EOP
)
EOF

Code:

plink.exe -ssh -l UserID -pw Password01 192.168.1.2 << EOF
$(
  while read port
  do
    cat << EOP
    config t
    int $port
    shut
    no shut
    exit
EOP
  done < ports.txt
)
    exit
    exit
EOF

Sorry, I'm feeling really dense. I really appreciate all the help on this forum!

The inner here-document is fed into cat in a loop. cat <<EOF is just a convenient method of printing multiple lines of text, they could have done the same with a bunch of "echo" statements. It all ends up as part of the outer here-document.

The outer here-document feeds the output of all of that into plink.

Hi, I can imagine it can be a bit difficult understand straight away. I'll try to explain what is going on:

Inside an (unquoted) here document you can have a variable reference. The contents of such a variable become part of that here document.

Code:

var=blabla
cat << EOF
This is the content of \$var: $var
EOF

output:

Code:

This is the content of $var: blabla

In this case instead of a variable I used command substitution ( $( .. ) ) which in itself contains another here document

Code:

cat << EOP
    config t
    int $port
    shut
    no shut
    exit
EOP

inside a while loop. In this here document there is another variable $port that gets expanded and becomes part, each time the here document is called.

So in thes case of your sample this command substitution will render:

Code:

    config t
    int Fa1/0/19
    shut
    no shut
    exit
    config t
    int Fa2/0/2
    shut
    no shut
    exit
    config t
    int Fa2/0/12
    shut
    no shut
    exit
    config t
    int Fa2/0/20
    shut
    no shut
    exit

Inside the outer here document, it is then followed by two additional exit statements.

@Scrutinizer thank you so much for taking the time to explain what's going on step by step. I had only seen command-substitution examples before when storing the output to variables; so, it took me some time to realize that it's also possible to just execute what's within $( ) directly as if it's just a single command.

@Corona688 thanks for helping me too!

You guys really make it a pleasure to learn bash scripting. I know it takes skill to be able to help explain how things work in easy-to-understand English.